This creates a project_sdk group that (mostly) follows the minilayout
group. This allows us to modify the pinned repositories of the
project_sdk manifest, without changing minilayout.

I said it mostly follows the minilayout checkout, because it also
includes 'rootdev', which has been problematic in project_sdk builds.

BUG=brillo:276
TEST=Used manually derived manifest based on project_sdk.
     Setup chroot.
     Setup board.
     emerge-panther_embedded --rebuilt-binaries virtual/target-os"
     emerge-panther_embedded --rebuilt-binaries virtual/target-os-test"

Change-Id: Ie0925e9794003b8f3bf452066158f7843ad20b31
Reviewed-on: https://chromium-review.googlesource.com/251204


Trybot-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>